1. Field of the Invention
The present invention relates to a communication apparatus and method, and a program in a computer readable medium, and more particularly to technologies of reducing a consumption power of each electronic apparatus for digital data communication or a whole system of electronic apparatuses.
2. Related Background Art
Technologies of configuring one communication system are being developed nowadays. This communication system connects AV apparatuses (e.g., digital cameras, digital video cameras) which process digitalized image signals and audio signals, to personal computers (hereinafter abbreviated as PC) and their peripheral apparatuses (e.g., printers, hard disk drives, CD-ROM drives), by using common communication interfaces, to thereby configure one communication system.
One example of communication interfaces used with such a communication system is IEEE 1394-1995 high performance serial bus standards (hereinafter called IEEE 1394 standards). A communication interface in conformity with IEEE 1394 standards (hereinafter called a 1394 interface) is a digital interface capable of transferring data at higher speed than conventional technologies and supporting a real time transfer of image and audio signals. The 1394 interface has several characteristics as in the following.
(1) The 1394 interface has two transfer modes, an isochronous transfer mode and an asynchronous transfer mode. The isochronous transfer mode ensures transmission/reception of data of a certain amount during each communication period (125 .mu.s), and is effective for real time transfer of image and audio signals. The asynchronous transfer mode transmits/receives data asynchronously with a communication cycle, and is effective for transmission/reception, when necessary, of control commands, file data, and the like. In each one communication cycle period, the asynchronous transfer mode has a lower priority over the isochronous transfer mode.
(2) The 1394 interface provides a connection configuration with a high degree of freedom. Specifically, a mixture of a daisy chain connection and a node branch connection is possible, allowing a configuration of a network with a high degree of freedom.
(3) The 1394 interface can automatically recognize the connection configuration of a network. Namely, when a new apparatus is added or removed or a power is turned on or off, the new network connection configuration is automatically recognized and IDs assigned to respective apparatuses are automatically set again.
(4) Since data transfer is serially executed between apparatuses, a thin communication cable can be used and a smaller connector can be used. The communication cable has a power feed pair cable so that power can be supplied to a 1394 interface of an apparatus whose main power was turned off. Therefore, even if the main power of an apparatus is turned off, communication over the network cannot be shut down.
The 1394 interface is, however, associated with the following problem.
In order for an apparatus equipped with a 1394 interface to achieve the above-described characteristic features, it is necessary to always feed a power to the whole circuitry of its own 1394 interface. Therefore, even during the period while an apparatus connected to the network is not necessary to communicate with another apparatus, it is required to always feed a power to its own 1394 interface wastefully. This problem becomes conspicuous when a portable apparatus (e.g., digital cameras, camera mounted digital video recorders) driven with an internal power source such as a battery is used. The 1394 interface of this portable apparatus consumes its internal power source more quickly.